In the production of a semiconductor such as an LSI or a ULSI or the production of a liquid crystal display panel, a pattern is formed by irradiating a semiconductor wafer or a liquid crystal master plate with light; if debris is attached to an exposure master plate, since the debris absorbs the light or refracts the light, there are problems that the replicated pattern is deformed, the edges become rough, or the background is stained black, thus impairing the dimensions, quality, appearance, etc. The ‘exposure master plate’ referred to in the present invention is a general term for lithography masks (also called simply ‘masks’) and reticles. The explanation below is given for a mask as an example.
These operations are usually carried out in a clean room, but even within a clean room it is difficult to always keep the exposure master plate clean, and a method is therefore employed in which a pellicle film that allows exposure light to easily pass through is affixed to the surface of the exposure master plate to act as a debris shield.
The pellicle is basically constituted of a pellicle frame and a pellicle film stretched over the frame. The pellicle film is formed from nitrocellulose, cellulose acetate, a fluorine-based polymer, etc., which allows exposure light (g rays, i rays, 248 nm, 193 nm, etc.) to easily pass through. The pellicle film is adhered by coating the upper end part of the pellicle frame with a good solvent for the pellicle film and air-drying or by means of an adhesive such as an acrylic resin, an epoxy resin, or a fluorine resin. Furthermore, in order to mount an exposure master plate, a lower end part of the pellicle frame is provided with a pressure-sensitive adhesion layer made of a polybutene resin, a polyvinyl acetate resin, an acrylic resin, a silicon resin, etc. and a reticle pressure-sensitive adhesive protecting liner for the purpose of protecting the pressure-sensitive adhesive layer.
The pellicle is installed so as to surround a pattern region formed on the surface of the exposure master plate. Since the pellicle is provided in order to prevent debris from becoming attached to the exposure master plate, this pattern region and a pellicle outer part are separated so that dust from the pellicle outer part does not become attached to the pattern face.
In recent years, the LSI design rule has shrunk to sub-quarter micron, and accompanying this the wavelength of the exposure light source is being shortened, that is, instead of g rays (436 nm) and i rays (365 nm) from the hitherto predominant mercury lamp, a KrF excimer laser (248 nm), an ArF excimer laser (193 nm), etc. are being used. As shrinkage advances, the flatness required for the mask and silicon wafer becomes more strict.
A pellicle is affixed to a mask in order to shield the pattern from debris after the mask is completed. When a pellicle is affixed to the mask, the mask flatness can sometimes change. When the mask flatness is degraded, as described above there is the possibility that problems such as defocusing will occur. Furthermore, when the flatness changes, the pattern shape drawn on the mask changes, and this brings about the difficulty that problems occur with the precision of superimposition of the mask.
There are several factors involved in the change in mask flatness due to a pellicle being affixed, but it has been found that the largest factor is the flatness of the pellicle frame.
In order to prevent deformation of a mask due to deformation of a pellicle frame, JP-A-2009-25562 (JP-A denotes a Japanese unexamined patent application publication) discloses reducing the cross-sectional area of the pellicle frame to 6 mm2 or less or using a material having a Young's modulus of 50 GPa or less in the pellicle frame.
Many pellicle frames have a rectangular cross-sectional shape, and JP-A-9-68793 discloses a pellicle frame for which the cross-section has a shape in which an upper end side of an inner peripheral face projects further inward than a lower end side.